U.S. patent application number 11/087733 was filed with the patent office on 2006-09-28 for high strength aluminum alloys and process for making the same.
Invention is credited to Charles E. Brooks, Ralph C. Dorward, Rob A. Matuska, Ray D. Parkinson, Mory Shaarbaf.
Application Number | 20060213591 11/087733 |
Document ID | / |
Family ID | 37033998 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060213591 |
Kind Code |
A1 |
Brooks; Charles E. ; et
al. |
September 28, 2006 |
High strength aluminum alloys and process for making the same
Abstract
High strength aluminum alloys based on the Al--Zn--Mg--Cu alloy
system preferably include high levels of zinc and copper, but
modest levels of magnesium, to provide increased tensile strength
without sacrificing toughness. Preferred ranges of the elements
include by weight, 8.5-10.5% Zn, 1.4-1.85 % Mg, 2.25-3.0% Cu and at
least one element from the group Zr, V, or Hf not exceeding about
0.5%, the balance substantially aluminum and incidental impurities.
In addition, small amounts of scandium (0.05-0.30%) are also
preferably employed to prevent recrystalization. During formation
of the alloys, homogenization, solution heat treating and
artificial aging processes are preferably employed.
Inventors: |
Brooks; Charles E.;
(Chandler, AZ) ; Dorward; Ralph C.; (Escalon,
CA) ; Parkinson; Ray D.; (Livermore, CA) ;
Matuska; Rob A.; (Heath, OH) ; Shaarbaf; Mory;
(Jackson, TN) |
Correspondence
Address: |
JONES, TULLAR & COOPER, P.C.
P.O. BOX 2266 EADS STATION
ARLINGTON
VA
22202
US
|
Family ID: |
37033998 |
Appl. No.: |
11/087733 |
Filed: |
March 24, 2005 |
Current U.S.
Class: |
148/551 ;
420/532 |
Current CPC
Class: |
C22C 21/10 20130101;
C22F 1/053 20130101 |
Class at
Publication: |
148/551 ;
420/532 |
International
Class: |
C22C 21/10 20060101
C22C021/10 |
Claims
1. An aluminum alloy product having high strength with good
toughness, containing by weight, 8.5-10.5% Zn, 1.4-1.85% Mg,
2.25-3.0% Cu, and at least one element from the group Zr, V, or Hf
not exceeding about 0.5%, the balance substantially aluminum and
incidental impurities.
2. The alloy product of claim 1, wherein said alloy contains about
0.05-0.2% Zr.
3. The alloy product of claim 1, wherein said alloy includes
0.05-0.30% Sc.
4. The alloy product of claim 3, wherein said alloy includes
0.05-0.20% Zr.
5. The alloy product of claim 1, wherein said alloy includes about
0.03-0.10% Si and 0.03-0.12% Fe.
6. The alloy product of claim 1, wherein the combined weight
percentages of Mg and Cu range from 4.1 to 4.5%.
7. The alloy product of claim 6, wherein the combined weight
percentages of Zn, Mg and Cu range from 13.0 to 14.5%.
8. The aluminum alloy product of claim 1, wherein said product is
selected from the group including sporting goods such as baseball
and soft ball bats, golf shafts, lacrosse sticks, tennis rackets,
and arrows; aerospace components such as wing plates, bulkheads,
fuselage stringers, and structural extrusions and forgings; and
ordnance parts such as sabots and missile launchers.
9. A process for making an aluminum alloy product containing
8.5-10.5% Zn, 1.4-1.85% Mg, 2.25-3.0% Cu, and at least one element
from the group Zr, V, or Hf not exceeding about 0.5%, the balance
substantially aluminum and incidental impurities, said method
including the steps of: casting said alloy product to form an alloy
ingot; and homogenizing said alloy ingot to minimize the amount of
low melting point eutectic phases therein by heating said ingot at
a heating rate of no more than 20.degree. F./hr. from a first
temperature at least about 20.degree. F. below the melting
temperature of said ingot to a second temperature of about
5.degree. F. below said melting temperature.
10. The process of claim 9, wherein said first temperature is about
30.degree. F. below said melting temperature.
11. The process of claim 9, wherein said first temperature is
selected to be about 870.degree. F. and said second temperature is
selected to be in the range of 885-890.degree. F.
12. The process of claim 9 where the alloy ingot is held at said
first temperature for at least 8 hours.
13. The process of claim 9, wherein said alloy contains 0.05-0.30 %
Sc.
14. The process of claim 9, wherein said alloy ingot is formed into
a shape of a finished product, solution heat treated at 870 to 900
degrees F. and then artificially aged in a first aging step by
being heated at 175-310 degrees F. for 3 to 30 hours.
15. The process of claim 14, wherein said product is exposed to a
second aging step by being heated at 310 to 360 degrees F. for 2 to
24 hours.
16. The process of claim 15, wherein said alloy product is exposed
to a third aging step by being heated at 175 to 300 degrees F. for
1 to 30 hours.
17. The process of claim 9, wherein said alloy ingot is formed into
a shape of a finished product, solution heat treated and then
artificially aged in a first aging step by being heated at 310 to
360 degrees F. for 2 to 24 hours.
18. The process of claim 17, wherein said alloy product is exposed
to a second aging step by being heated at 175 to 300 degrees F. for
1 to 30 hours.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application contains subject matter that is related to
the subject matter set forth in U.S. application Ser. No.
10/829,391, which was filed on Apr. 22, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates, in general, to a high
strength aluminum alloy based on the Al--Zn--Mg--Cu alloy system
and a process for forming the same. Although not limited thereto,
the alloys are particularly suited for use in sporting goods and
aerospace applications.
[0004] 2. Description of the Background Art
[0005] The highest strength aluminum alloys known at this time are
based on the aluminum-zinc-magnesium-copper system. Commercial
high-strength alloys currently being produced include AA7055
(nominally 8% Zn-2% Mg-2.2% Cu-0.10% Zr), AA7068 (nominally 7.8%
Zn-2.5% Mg-2.0% Cu-0.10% Zr) and a Kaiser Aluminum alloy designated
K749 (nominally 8% Zn-2.2% Mg-1.8% Cu-0.14% Zr). From the published
phase relationships at 860.degree. F. for an alloy containing 8%
Zn, one can note that K749 is near a phase boundary, while the
other two alloys are in multiple phase fields. In the latter case
all the alloying elements are not in solid solution at 860.degree.
F., and are not only unavailable for age hardening, but the
undissolved phases remaining after heat treatment detract from
toughness. Although solution heat treating at a higher temperature
than 860.degree. F. will dissolve more of the solute, care has to
be taken to ensure that the alloy does not undergo eutectic
melting, which is a common problem in commercially cast alloys that
have locally enriched regions as a result of microsegregation that
occurred during casting.
[0006] There is a need in many applications, such as sporting goods
and aerospace applications, for even stronger alloys based on the
aluminum-zinc-magnesium-copper system that do not sacrifice
toughness. However, this requirement presents a problem because, in
general, as the tensile strength of an aluminum alloy is increased,
its toughness decreases.
SUMMARY OF THE INVENTION
[0007] The present invention addresses the foregoing need in a
number of ways. More particularly, there are three distinct avenues
for increasing an alloy's strength while maintaining its toughness:
rich alloy chemistries; processing to maximize alloying
effectiveness; and preventing recrystallization. Rich alloys
provide more solute, which is potentially available for age
hardening to higher strength levels; effective processing ensures
that the solute is available for strengthening and not out of
solution as second phases, which detract from fracture toughness;
and maintaining an unrecrystallized microstructure optimizes both
strength and toughness.
[0008] To provide increased tensile strength without sacrificing
toughness through the use of rich chemistries, the present
invention comprises aluminum alloys based on the Al--Zn--Mg--Cu
alloy system that preferably include high levels of zinc and
copper, but modest levels of magnesium. As an option, small amounts
of scandium can also be employed to prevent recrystallization. Each
of the alloys preferably includes at least 8.5% Zn and 2.25% Cu by
weight. Higher levels of each of these elements up to about 10.5%
Zn and 3.0% Cu can be used. However, modestly lower amounts of Mg
(max 1.85%) are preferably used to allow higher levels of the Cu.
The preferred ranges of all elements in the alloys include by
weight, 8.5-10.5% Zn, 1.4-1.85 % Mg, 2.25-3.0% Cu, and at least one
element from the group Zr, V, or Hf not exceeding about 0.5%, the
balance substantially aluminum and incidental impurities. In the
preferred embodiments, 0.05-0.30% Sc is also included in the alloys
to prevent recrystallization. Additionally, it has been found that
toughness decreases as the total weight percentage of magnesium and
copper increases. Experiments have established that the ideal range
of these two elements be between 4.1 and 4.5% combined. Still
further, the total weight percent of Zn, Cu and Mg is ideally
between 13.0 and 14.5%.
[0009] To maximize alloying effectiveness during formation of the
alloys, a homogenization process is preferably employed after alloy
ingot casting in which a slow rate of temperature increase is
employed as the alloy is heated as near as possible to its melting
temperature. In particular, for the last 20-30.degree. F. below the
melting temperature, the rate of increase is limited to 20.degree.
F./hr. or less to minimize the amount of low melting point eutectic
phases and thereby further enhance fracture toughness of the alloy.
Once the ingot is formed into finished shape using extrusion and
rolling steps, for example, the product is preferably solution heat
treated at 870 to 900 degrees F. and then artificially aged. The
aging process can be carried out by exposing the product to a one,
two or three step heat treatment process. In the first step, the
product is exposed to a temperature range of 175-310 degrees F. for
3 to 30 hours. In the optional second step, the first step is
followed by heating at 310 to 360 degrees F. for 2 to 24 hours.
Finally, in the third optional step, the product is heated at 175
to 300 degrees F. for 1 to 30 hours. As a still further option, the
second and third aging steps can be used without the first aging
step.
[0010] The foregoing alloys and processing operations enhance the
properties of the Al--Zn--Mg--Cu alloy system, such that they can
be more effectively employed in numerous applications. Specific
products or items in which the subject alloys can be employed
include, among others, sporting goods including baseball and soft
ball bats, golf shafts, lacrosse sticks, tennis rackets, and
arrows; and aerospace application including aerospace components
such as wing plates, bulkheads, fuselage stringers, and structural
extrusions and forgings; and ordnance parts such as sabots and
missile launchers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The features and advantages of the present invention will
become apparent form the following detailed description of a
preferred embodiment thereof, taken in conjunction with the
accompanying drawings, in which:
[0012] FIG. 1 is a graph depicting T6 strength (YTS and UTS) as a
function of the total alloy content in weight percent for a number
of sample alloys formed in accordance with the preferred
embodiments;
[0013] FIG. 2 is a graph depicting fracture toughness as a function
of combined percentages of Cu and Mg for sample alloys formed in
accordance with the preferred embodiments;
[0014] FIG. 3 is an equilibrium diagram which depicts the phase
relationships at 885.degree. F. as a function of percentages of Cu
and Mg for an alloy formed in accordance with the preferred
embodiments that contains 9% Zn;
[0015] FIG. 4 is a graph illustrating the effect of the ratio of Mg
to Cu on fracture toughness for the alloys formed in accordance
with the preferred embodiments;
[0016] FIG. 5 is a graph depicting second phase volume percent as a
function of heating rate in a formation process for Alloy AA7068;
and
[0017] FIG. 6 is a graph illustrating the effect of scandium on
strength of an Al-8% Zn-2.2% Mg-1.9% Cu alloy.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] The following examples illustrate how alloy modifications
and efficient processing operations can be used to enhance the
properties of the Al--Zn--Mg--Cu alloy system in accordance with
the preferred embodiments of the present invention, such that they
can be more effectively utilized in sporting goods and aerospace
applications.
EXAMPLE 1
[0019] A heretofore unexplored region of the Al--Zn--Mg--Cu alloy
system consists of compositions comprising about 9% to 10% zinc,
2.2% to 2.8% copper, and 1.6% to 2.0% magnesium. The alloy
compositions listed in Table 1 were cast as 9-in. diameter billets:
note that all these alloys contain about 0.05% scandium, an element
which in combination with zirconium is effective in preventing
recrystallization. TABLE-US-00001 TABLE 1 Alloy compositions
Percent by Weight Alloy Si Fe Cu Mg Zn Zr Sc 179 0.04 0.07 2.47
1.83 8.87 0.14 0.06 180 0.04 0.09 2.71 1.89 8.95 0.13 0.06 189 0.04
0.08 2.14 1.89 8.60 0.12 0.05 190 0.03 0.09 2.31 1.86 9.21 0.13
0.05 191 0.03 0.11 2.35 1.81 9.63 0.13 0.05 192 0.04 0.10 2.33 1.87
10.13 0.12 0.05 200 0.04 0.09 2.58 1.64 8.84 0.12 0.05 202 0.04
0.12 2.46 1.66 8.87 0.13 0.05 203 0.04 0.10 2.69 1.78 8.94 0.13
0.05 204 0.03 0.10 2.88 1.58 8.78 0.12 0.05 209 0.04 0.08 2.64 1.49
8.78 0.14 0.05 213 0.03 0.07 2.42 1.63 9.65 0.13 0.05 214 0.03 0.09
2.56 1.44 9.50 0.14 0.05 215 0.04 0.09 2.57 1.73 9.82 0.12 0.05 216
0.03 0.10 2.81 1.60 9.65 0.13 0.05
[0020] The billets were homogenized at 880 F. (F means degrees
Fahrenheit) and extruded to seamless 4-in. diameter tubes with a
0.305 in. wall thickness. The extrusions were solution heat treated
at 880 F., quenched in cold water and "peak" aged to the T6 temper
(24-hr soak at 250 F.). They were tested for tensile properties in
the longitudinal direction and sections from all of the extrusions
were cut and flattened to pieces about 12'' square, which were also
solution heat treated at 880 F., quenched in cold water and peak
aged. These flattened sections were tested for fracture toughness
(ASTM B645) in the T-L orientation. The tensile and fracture
toughness properties are listed in Table 2. TABLE-US-00002 TABLE 2
Tensile and fracture toughness properties Strength Toughness (ksi)
(ksi rt.in) % Zn % Cu % Mg UTS YTS Kq Kp 8.60 2.14 1.89 97.1 88.5
25.2 30.5 9.21 2.31 1.86 100.1 93.9 22.4 27.5 9.63 2.35 1.81 99.9
94.2 20.9 25.4 10.13 2.33 1.87 103.2 97.8 21.2 24.0 8.87 2.47 1.83
101.1 92.2 20.9 23.9 8.95 2.71 1.89 102.9 93.7 20.1 20.5 8.84 2.58
1.64 98.6 93.8 23.1 25.8 8.87 2.46 1.66 98.4 92.8 25.3 22.2 8.94
2.69 1.78 100.0 94.2 24.2 22.4 8.78 2.88 1.58 99.1 93.8 24.8 21.9
8.78 2.64 1.49 96.4 91.9 24.8 22.9 9.65 2.42 1.63 100.3 96.3 24.7
21.3 9.50 2.56 1.44 98.5 94.9 26.2 21.2 9.82 2.57 1.73 102.6 98.2
21.9 18.2 9.65 2.81 1.60 100.6 97.1 20.0 18.4
[0021] As can be seen from Table 2, tensile yield strengths well in
excess of 90 ksi were obtained in most of the alloys, with two
compositions achieving about 98 ksi. As shown in FIG. 1, strength
correlated well with the total alloy content, with each wt. pct.
adding about 4.8 ksi to the yield strength. The equilibrium phase
relations at the homogenizing and solution heat treatment
temperature explain the reason for this behavior. FIG. 3 shows how
the compositions listed in Table 1 relate to the magnesium and
copper solubility limits at 885F for alloys containing a nominal
zinc level of 9%. Compositions lying below the demarcation line
between the solid solution and the Al+S phase regions (i.e., the
solvus) are single phase alloys, which have superior fracture
toughness values for a given strength level, compared to those in
the 2-phase region. The best combinations of strength and toughness
are associated with alloys near the solvus line, which is why the
2.7% Cu/1.9% Mg composition has a relatively low toughness level.
The preferred compositions therefore lie within the dashed lines
that run approximately parallel to the solvus. These relationships
are defined by controlling the total copper plus magnesium
concentrations between 4.1 % and 4.5%.
[0022] Although the properties described above were obtained with a
"standard" T6 temper aging treatment by exposing the shaped
products to heat of between 175 and 310 F. for 3 to 30 hours (24 hr
at 250 F. was specifically used), as with most Al--Zn--Mg--Cu
alloys, other practices may also be advantageous, depending on the
desired combination of properties. For example, a tube from
composition #213, when drawn to a tube 2.625'' in diameter with a
0.110'' wall thickness and aged by a 2-step practice of 8 hr at 250
F. plus 4 hr at 305 F. had yield and tensile strengths of 100.9 ksi
and 102.6 ksi, respectively. Similarly, the subject alloy can be
over aged beyond peak strength in a second step at temperatures in
the 310-360 F. temperature range for 2 to 24 hours to provide a
desirable combination of strength and corrosion resistance. Another
preferred embodiment includes a final aging treatment in a third
step at a lower temperature in the range 175-300 F. for 1 to 30
hours, which provides an additional strength benefit with no loss
in corrosion properties. As yet another alternative, the alloy can
be subjected only to the aforementioned second and third aging
steps by skipping the first step.
EXAMPLE 2
[0023] To compare the invention alloy with other commercial
high-zinc alloys such as AA7036, AA7056 and AA7449, which have
higher Mg/Cu ratios in the range 1.0 to 1.4, the following alloys
were prepared as described in Example 1. TABLE-US-00003 TABLE 3
Compositions of Comparative Alloys Percent by Weight Alloy No. Si
Fe Cu Mg Zn Zr Sc 36 0.03 0.06 1.91 2.17 9.02 0.15 0.05 39 0.04
0.05 1.28 2.74 9.02 0.13 0.06 43 0.03 0.03 1.44 2.62 9.04 0.13 0.05
47 0.04 0.06 1.59 2.34 8.95 0.14 0.06
[0024] The yield strengths and toughness values for these alloys
are listed in the following table. TABLE-US-00004 TABLE 4
Mechanical Properties of Comparative Alloys Mg/Cu Yield Kpmax Alloy
Ratio %(Mg + Cu) Strength (ksi) (ksi rtin.) 36 1.14 4.08 94.9 24.5
47 1.47 3.93 93.9 22.7 43 1.77 3.99 93.9 21.3 39 2.14 4.02 92.7
20.2
[0025] FIG. 4 compares the toughness levels of these alloys on the
basis of Mg/Cu ratio with the invention alloys, using those
compositions that have similar strength levels (93-95 ksi) and
total Mg+Cu contents (4.0-4.2%).
EXAMPLE 3
[0026] As noted earlier it is important that undissolved second
phases do not remain after processing so that fracture toughness
can be maximized. This is especially important in alloys that are
rich in alloy content, and lie near an equilibrium solvus phase
boundary. To illustrate how homogenizing practice can affect the
amount of such undissolved phase(s), samples of as-cast AA7068
alloy billet were heated from 850 F. at various rates in a
differential scanning calorimeter (DSC), and the energy associated
with eutectic melting, which started at about 885 F. was measured.
This energy measurement is directly proportional to the amount of
undissolved second phase remaining at the incipient melting point,
and the relationship between these factors has been determined by
quantitative microscopy. FIG. 5 shows how heating rate affects the
amount of this phase as determined from the DSC data.
[0027] Note that a slow heating rate of about 10 F./hr reduces the
amount of second phase to a level below 1 vol.%. One would expect
that a .about.5 F/hr heating rate would reduce the "soluble"
portion to near zero. We also note that for heating rates of 10-20
F./hr, the volume fraction of undissolved eutectic is no greater
than the amount of insoluble Fe-containing constituent (independent
of heating rate or homogenization temperature) at a nominal 0.12%
Fe level (approx. 1 vol.%).
EXAMPLE 4
[0028] It has been recognized for a number of years that scandium
in combination with zirconium is an effective recrystallization
inhibitor. A Russian review article states "it is desirable to add
scandium to aluminum alloys in a quantity from 0.1 to 0.3% together
with zirconium (0.05-0.15%)". However, "the greatest effect . . .
is observed for alloys not containing alloy elements combining with
scandium in insoluble phases . . . ; with a limited copper content
[scandium combines with copper] alloying with scandium together
with zirconium of Al--Zn--Mg--Cu and Al--Cu--Li alloys is
possible". As such, "commercial alloys based on Al--Zn--Mg--Sc--Zr
(01970, 01975) have been developed".
[0029] Two potential drawbacks to scandium additions to 7XXX alloys
containing about 2% copper are evident:
[0030] 1) the copper level is high enough to combine with scandium,
thereby rendering it ineffective, and
[0031] 2) the high price of scandium; at the 0.2% level it would
add about $10 a pound to the cost of the aluminum alloy.
[0032] It would therefore be economically and technically
attractive if scandium levels could be effectively used below those
recommended in the Russian literature.
[0033] Alloys of the compositions listed in the following table
were prepared as 5'' diameter billets, which were processed as
described below. Although the sample alloys contained more Mg and
less Cu than the preferred alloys discussed previously, it is
believed that the effect of Sc addition to the alloys would be
essentially the same for the preferred alloys. TABLE-US-00005 Alloy
% by wt. No. Si Fe Cu Mg Zn Zr Sc A 0.03 0.04 1.95 2.20 8.07 0.11
0.00 B 0.03 0.05 1.86 2.17 8.05 0.00 0.22 C 0.03 0.05 1.89 2.18
8.09 0.11 0.06 D 0.03 0.04 1.84 2.12 8.11 0.12 0.11 E 0.03 0.05
1.95 2.18 8.08 0.11 0.22
[0034] The ingots were homogenized at 875 F. using a 50 F./hr
heating rate and air cool, and then reheated to 800 F. and extruded
to a 0.25'' by 3'' flat bar. Sections of each extrusion were
annealed at 775 F. for 3 hr, cooled 50 F./hr to 450 F., held 4 hr
and cooled 50 F./hr to room temperature. The sections were then
cold rolled to 0.040'' sheet using five pass reductions (84% total
reduction). The sheets were solution heat treated at 885 F. for 30
min, quenched in cold water, and then aged to the peak strength
condition (10 hr at 305 F.). The as-extruded bars were also heat
treated similarly and both products were tested for transverse
tensile properties, as listed below. The specific effects of
scandium on strength are also shown in FIG. 6. TABLE-US-00006 Alloy
UTS (ksi) Yield Strength (ksi) No. % Zr % Sc Extrusion Sheet
Extrusion Sheet A 0.11 0 94.7 90.7 91.4 87.8 B 0 0.22 88.2 92.0
86.1 88.4 C 0.11 0.06 95.7 97.1 92.2 93.3 D 0.12 0.11 95.2 96.6
92.2 93.3 E 0.11 0.22 94.5 96.5 91.1 92.5
[0035] A number of points are evident from these results: [0036] 1.
The strongest alloy in both extrusion and sheet form contains 0.06%
Sc (with 0.11% Zr) [0037] 2. At the 0.1% Zr level, 0.06% Sc is
effective in raising the strength of the sheet product by about 6
ksi. [0038] 3. 0.22% Sc in the absence of zirconium raises the
strength of the sheet product by only 1 ksi, and lowers the
extrusion strength by about 6 ksi. The effectiveness of only 0.06%
Sc in preventing recrystallization was confirmed by comparing the
microstructures of the sheet products containing (a) 0.11% Zr, (b)
0.11% Zr+0.06% Sc, and (c) 0.22% Sc (no Zr). In view of the
foregoing, the preferred range in the alloys for Sc is 0.05-0.30%,
with a more preferred range of 0.05-0.10%.
[0039] Although the present invention has been described in terms
of a number of preferred embodiments and variations thereon, it
will be understood that numerous additional variations and
modifications may be made without departing from the scope of the
invention. Thus, it is to be understood that within the scope of
the appended claims, the invention may be practiced otherwise than
as specifically described above.
* * * * *